Three dimensional scatterometry uses a diffraction based reflectometer to measure embedded details and critical profile parameters of complex three dimensional semiconductor structures. Reliable measurements of such details make three dimensional scatterometry an important instrument for both device development and process control. Three dimensional scatterometry combines non-contact optical technology with advanced modeling software to provide accurate measurements resulting in a three dimensional metrology system.
Spectroscopic ellipsometry and scatterometry are used to measure low k dielectric film thickness formed above a 50×50 micrometers (μm) copper (Cu) pad (also referred to as a one-dimensional metrology target) in a back end of line (BEOL) interconnect. However, Cu pad thinning due to pattern dependent Cu chemical mechanical polishing (CMP) causes inconsistent light transmissibility resulting in unstable ultra-low k (ULK) dielectric thickness measurement. For advanced technology nodes, (e.g., 20 nm, 14 nm and beyond), based upon process requirements, the design of Cu trench height has been drastically reduced, often less than 60 nm).
Measurement of remaining Cu height on 20 nm technology node wafers reveals that Cu height at conventional one-dimensional film thickness metrology structures can vary by up to 80% within wafer. This observed within wafer variation in Cu height of the conventional one-dimensional film thickness metrology structures demonstrates the probability of measurement noise due to penetration of light through the Cu and the difficulty in utilizing a uniform ellipsometry model for the Cu for both center and edge measurement locations.
A need therefore exists for methodology and an apparatus for enabling measurement of a ULK dielectric film stack above a three-dimensional Cu trench grating, with improved measurement repeatability and without sensitivity to underlying Cu trench height.